v3.26.1
Agreements and Related Party Transactions - Additional Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Sep. 30, 2025
Related Party Transaction [Line Items]          
Annual rate of base management fee     1.00%    
Base management fee $ 6,400 $ 5,600 $ 13,200 $ 10,900  
Base management fee calculation description     The base management fee is calculated based on the average adjusted gross assets at the end of the two most recently completed calendar quarters, and appropriately adjusted for any share issuances or repurchases during the current calendar quarter.    
Incentive fee quarter hurdle rate     1.75%    
Incentive fee annual hurdle rate     7.00%    
Incentive fees on net investment income 6,400 6,300 $ 13,100 13,800  
Description of incentive fee on net investment income     We pay the Investment Adviser an incentive fee with respect to our Pre-Incentive Fee Net Investment Income in each calendar quarter as follows: (1) no incentive fee in any calendar  quarter in which our Pre-Incentive Fee Net Investment Income does not exceed the hurdle rate of 1.75%, (2) 50% of our Pre-Incentive Fee Net Investment Income with respect to that portion of such Pre-Incentive Fee Net Investment Income, if any, that exceeds the hurdle rate but is less than 2.9167% in any calendar quarter (11.67% annualized) (we refer to this portion of our Pre-Incentive Fee Net Investment Income (which exceeds the hurdle but is less than 2.9167%) as the “catch-up,” which is meant to provide our Investment Adviser with 20% of our Pre-Incentive Fee Net Investment Income, as if a hurdle did not apply, if this net investment income exceeds 2.9167% in any calendar quarter), and (3) 20% of the amount of our Pre-Incentive Fee Net Investment Income, if any, that exceeds 2.9167% in any calendar quarter.    
Incentive fee on capital gains percentage     20.00%    
Incentive fee on capital gains accrued 0 0 $ 0 0  
Incentive fee on unrealized and realized capital gains accrued 0 0 0 0  
Administration fees 1,200 1,200 2,400 2,400  
Due from affiliates 0   0   $ 700
Total Payable 900   900   0
Other Related Party          
Related Party Transaction [Line Items]          
Net investment income 0 0 0 0  
PennantPark Senior Secured Loan Fund I LLC          
Related Party Transaction [Line Items]          
Gain on Sale of Investments (100) 0 (200) (100)  
Administration fees [1] 795 717 1,553 1,385  
Net investment income 56,900 52,900 189,400 240,600  
Receivable from the Administraton 100   100   300
Total Payable 271,600   271,600   $ 271,600
PennantPark Senior Secured Loan Fund II LLC          
Related Party Transaction [Line Items]          
Gain on Sale of Investments (500)   (1,000)    
Net investment income 148,100 0 344,600 0  
Administrator          
Related Party Transaction [Line Items]          
Administration fees $ 700 $ 500 1,400 $ 900  
Incentive Fee Payment Condition One          
Related Party Transaction [Line Items]          
Incentive fees on net investment income     $ 0    
Incentive Fee Payment Condition One | Maximum          
Related Party Transaction [Line Items]          
Incentive fee quarter hurdle rate     1.75%    
Incentive Fee Payment Condition Two          
Related Party Transaction [Line Items]          
Pre-incentive fee net investment income percentage     50.00%    
Incentive Fee Payment Condition Two | Maximum          
Related Party Transaction [Line Items]          
Incentive fee quarter hurdle rate     2.9167%    
Incentive fee annual hurdle rate     11.67%    
Incentive Fee Payment Condition Two | Minimum          
Related Party Transaction [Line Items]          
Incentive fee quarter hurdle rate     2.9167%    
Pre-incentive fee net investment income percentage     20.00%    
Incentive Fee Payment Condition Three          
Related Party Transaction [Line Items]          
Pre-incentive fee net investment income percentage     20.00%    
Incentive Fee Payment Condition Three | Minimum          
Related Party Transaction [Line Items]          
Incentive fee quarter hurdle rate     2.9167%    
[1] No management or incentive fees are payable by PSSL. If any fees were to be charged, they would be separately disclosed in the Consolidated Statement of Operations. PSSL pays the Administrator an annual fee of 0.25% of average gross assets under management on a quarterly basis.